An electronic throttle valve system for electronically controlling the opening of a throttle valve to adjust the amount of intake air to an engine (internal combustion engine) can reduce emission and fuel consumption and has been used in some four-wheeled motor vehicles.
Such an electronic throttle valve system is equipped with a function which stops the driving of the throttle valve by an electric motor and returns the throttle valve to the fully closed position with the urging force of a spring when the control system has a failure. The engine is thereby maintained in such a state that a failure operation can be conducted and the vehicle can be driven to a place.
When a bypass line is provided so that a certain amount of air can be sucked into the engine even when the throttle valve is returned to the fully closed position by the urging force of a spring, the engine can be maintained in such a state that a failure operation can be conducted.
Patent Document 1 discloses a method of rotating a throttle valve and holding it in a predetermined opening position by urging forces of a spring for urging the throttle valve in the closing direction and another spring for urging the throttle valve in the opening direction without a bypass line.
The speed, at which a throttle valve is rotated in the closing direction by the urging force of a spring when the control system has a failure, is very high. Thus, the output of the engine is rapidly decreased. In the case of a four-wheeled passenger vehicle, a driver does not feel a change in the behavior of the vehicle even when the engine output is rapidly decreased since the vehicle is heavy in weight. In the case of a two-wheeled motor vehicle which is light in weight, however, the rider feels a change in the behavior of the vehicle.
Patent Document 2 discloses a method of closing a throttle valve gently by applying a resistance to the rotation of the throttle valve urged in the closing direction by a spring to prevent rapid rotation of the throttle valve. A rapid decrease in the engine output can be thus avoided and the vehicle does not make jerky movements even when running in low gear. As the means for applying a resistance to the rotation of the throttle valve (damping mechanism), an electronic damper using an electroviscous fluid is used.
Patent Document 3 discloses an example in which an electronic throttle valve system is applied to a two-wheeled motor vehicle.    Patent document 1: JP-A-2003-201866    Patent document 2: JP-A-Hei 6-248979    Patent document 3: JP-A-2002-106368